A sensor has been available which detects the presence of a potentially explosive gas, a toxic gas, or the like, or its quantitative concentration. This sensor adsorbs a particular type of molecules contained in the gas to detect whether or not molecules have been actually adsorbed, or the amount of molecules adsorbed. The sensor thus detects the presence or concentration of any gas. Such a sensor is installed in facilities, equipment, apparatuses, and the like which handle gas, to control gas leakage or amount.
Much effort has been made to develop fuel cells in recent years. Since fuel cells use hydrogen, it is preferable to monitor hydrogen stations as well as vehicles, apparatuses, instruments, and the like which use fuel cells, for hydrogen leakage. The above sensor can be used for these applications.
The sensor adsorbing a particular type of molecules to detect whether or not molecules have been actually adsorbed, or the amount of molecules adsorbed can be used not only for the above applications but also for the following: for example, analysis of freshness or components of foods, environmental control for providing and maintaining a comfortable space, and sensing of conditions of a living organism such as a human body.
Some such sensors may be based on a scheme using a cantilever. In this scheme, a molecule adsorption film (sensitive film) is provided on the cantilever to adsorb a particular type of molecules. Adsorption of molecules is detected on the basis of a change in the condition of the cantilever which occurs when molecules are adsorbed on the molecule adsorption film. Adsorption of molecules on the molecule adsorption film changes the stress of the molecule adsorption film. This changes the deflection amount of the cantilever, which makes it possible to detect that the particular type of molecules has been adsorbed. Further, the adsorption of molecules increases the mass of the molecule adsorption film, thus changing the resonance frequency of a system consisting of the cantilever and the molecule adsorption film. Accordingly, adsorption of the particular types of molecules can also be detected on the basis of the change in the resonance frequency (see, for example, Non-Patent Document 1).
It has already been reported that the scheme enables the realization of sensing of hydrogen gas by using an adsorption film made of platinum or palladium to adsorb hydrogen molecules, sensing of alcohol components using a PMMA polymer, sensing of smells of foods, and the like. With a method of detecting a change in the resonance frequency of the cantilever, when particular molecules are adsorbed to the adsorption film to very slightly change the mass of the film, the resonance frequency of the cantilever, which has a large resonance Q value, changes by very sensitively responding to the change in the mass. This enables improved sensing.
With the conventional method of sensing gas using a change in the resonance frequency of the cantilever, the sensor itself can be comprised of a cantilever of size several tens of μm to several hundred μm which is produced by a micromachining technology. This method is thus characterized by enabling a reduction in the size of the sensor and enabling the resonance Q value to be increased as previously described. Consequently, this configuration is excellent in size reduction and improved sensitivity.
When such a cantilever is used for sensing, to allow the cantilever to be vibrated (driven), a piezoelectric layer made of any of various piezoelectric materials and an electrode layer made of metal are provided on the surface of the cantilever main body formed of a silicon-containing material such as SiO2 (silicon dioxide). Then, the electrode layer applies a voltage to the piezoelectric layer to vibrate the cantilever. A change in the resonance frequency of the cantilever is then monitored.
As the piezoelectric material forming the piezoelectric layer, what is called a ferroelectric thin film formed of a material containing Pb (lead), Zr (zirconium), and Ti (titanium) has been gathering much attention.
Non-Patent Document 1: Suman Cherian, Thomas Thundat, “Determination of adsorption-induced variation in the spring constant of a microcantilever”, Applied Physics Letter, 2002, Vol. 80, No. 12, pp. 2219 to 2221).
However, there has constantly been a demand for an increase in the sensitivity of a sensor using a vibrator the vibration property of which is changed by adhesion of a very small mass to the sensor as described above.
The piezoelectric layer made of the ferroelectric thin film and the electrode layer are provided on the surface of the cantilever. The piezoelectric layer and the electrode layer themselves suffer attenuation, resulting in a loss in the vibration energy of the cantilever. This reduces the Q value of the cantilever and thus the sensitivity of the sensor. In this regard, the conventional technique still has room for improvement.